Voltage source inverter feed AC motor drives have become increasingly popular in general industrial applications, as well as in transportation vehicles such as electrical propulsion systems. In such applications, a wide operating speed range above the base speed (e.g., a high speed cruise) is often required. The recently emerging “more electrical” aircraft concept has also created more demand for AC motor drives in aerospace applications, such as for supplying engine starter, fan, and pump loads. Because of the limited DC bus voltage on aircraft and high output power rating requirement, some of these drive systems must be designed to operate at field weakening mode even at the rated operating point to achieve maximum voltage/current utilization and high efficiency operation. This makes field weakening control a critical part of the motor controller design.
When motor speed is lower than base speed, the inverter can provide enough voltage to support motor back EMF, so that field weakening is not required. When motor speed is higher than base speed, however, motor back EMF will exceed the inverter output voltage capability unless field weakening is applied. Thus, field weakening must be implemented to reduce the effective back EMF to achieve high-speed operation above base speed.
One basic field-weakening technique, such as the one applied in U.S. Pat. No. 6,407,531 issued to Walters et al. on Jun. 18, 2002, relies on a look up table. This kind of technique, however, requires that a large quantity of data be created off line and stored in the memory to achieve optimal field weakening control under any DC link voltage and any load conditions. Furthermore, sufficient margin must be factored in for parameter variation and the extra voltage needed during transition state. As such, the inverter output voltage capability cannot be fully utilized, which is a significant drawback for aerospace applications because it is directly related to the inverter size and weight.
Another approach to flux weakening is to calculate, on-line, the field weakening current from motor equations. Such an approach is described in U.S. Pat. No. 5,739,664 issued to Deng et al. on Apr. 14, 1998 and U.S. Pat. No. 6,504,329 issued to Stancu et al. on Jan. 7, 2003. These approaches, however, are very sensitive to uncertainties related to the system parameters and equations will be very complex for systems with an AC side output LC filter. A sufficient margin must be factored in to ensure stable system operation even with parameter variation. Thus, inverter output capability cannot be fully utilized.
A known field weakening control scheme is a close loop method, which does not use machine parameters for calculations in the field weakening operation. This control scheme should be able to adjust field-weakening current automatically during transient and steady state according to DC link voltage and load conditions. U.S. Pat. No. 5,168,204 issued to Schauder on Dec. 1, 1992, U.S. Pat. No. 6,288,515 issued to Hiti et al. on Sep. 11, 2001 and the paper authored by J. H. Song, J. M. Kim and S. K. Sul, entitled “A New Robust SPMSM Control to Parameter Variations in Flux Weakening Region,” Proc. IECON'96, pp. 1193–1198, 1996, provide techniques that possess these features. These techniques adjust field-weakening current according to the inverter output voltage amplitude. There is no need for machine parameters but the choice of parameters in the field-weakening loop is still critical for the stability of the system. Because such techniques are close-loop based, during transition both d-axis and q-axis current loops lose control due to the shortage of voltage and over modulation will also occur, which will cause high frequency resonance for systems with AC side output LC filters. Unfortunately, many drive systems in aerospace applications require LC filters for the tough EMI requirements and the long cable between inverter and motor.